Process for C8 alkylaromatic isomerization

ABSTRACT

A process for isomerizing ethylbenzene into xylenes such as para-xylene using a zeolitic catalyst system based on low Si/Al 2  MTW-type zeolite that preferably is substantially free of mordenite. The catalyst may be bimetallic where the two metals are platinum and tin.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In Part of applications Ser. No.10/749,156 and Ser. No. 10/749,179, both filed Dec. 30, 2003, both nowabandoned, the contents of each are hereby incorporated by reference inits entirety.

FIELD OF THE INVENTION

The present invention relates to catalytic processes for theisomerization of xylenes and for the conversion of ethylbenzene in thepresence of hydrogen.

BACKGROUND OF THE INVENTION

The xylenes, para-xylene, meta-xylene and ortho-xylene, are importantintermediates that find wide and varied application in chemicalsyntheses. Para-xylene upon oxidation yields terephthalic acid that isused in the manufacture of synthetic textile fibers and resins.Meta-xylene is used in the manufacture of plasticizers, azo dyes, woodpreservers, etc. Ortho-xylene is feedstock for phthalic anhydrideproduction.

Xylene isomers from catalytic reforming or other sources generally donot match demand proportions as chemical intermediates, and furthercomprise ethylbenzene, which is difficult to separate or to convert.Para-xylene in particular is a major chemical intermediate with rapidlygrowing demand, but amounts to only 20-25% of a typical C₈ aromaticsstream. Adjustment of isomer ratio to demand can be effected bycombining xylene-isomer recovery, such as adsorption for para-xylenerecovery, with isomerization to yield an additional quantity of thedesired isomer. Isomerization converts a non-equilibrium mixture of thexylene isomers that is lean in the desired xylene isomer to a mixtureapproaching equilibrium concentrations.

In general, these xylene isomerization processes comprise contacting thexylene isomer sought to be isomerized with an isomerization catalystunder isomerization conditions. Various catalysts have been proposed forxylene isomerization. These catalysts include molecular sieves,especially molecular sieves contained in a refractory, inorganic oxidematrix. U.S. Pat. No. 4,899,012 discloses an alkylaromatic isomerizationprocess based on a bimetallic pentasil-type zeolitic catalyst systemthat also produces benzene. U.S. Pat. No. 4,962,258 discloses a processfor liquid phase xylene isomerization over gallium-containing,crystalline silicate molecular sieves as an improvement overaluminosilicate zeolites ZSM-5, ZSM-12 (MTW-type), and ZSM-21 as shownin U.S. Pat. No. 3,856,871. The '258 patent refers to borosilicate work,as exemplified in U.S. Pat. No. 4,268,420, and to zeolites of the largepore type such as faujasite or mordenite. U.S. Pat. No. 5,744,673discloses an isomerization process using beta zeolite and exemplifiesthe use of gas-phase conditions with hydrogen. U.S. Pat. No. 5,898,090discloses an isomerization process using crystallinesilicoaluminophosphate molecular sieves. U.S. Pat. No. 6,465,705discloses a mordenite catalyst for isomerization of aromatics that ismodified by an IUPAC Group III element. U.S. Pat. No. 6,143,941, forinstance, discloses oil dropped catalyst structures for xyleneisomerization in which various molecular sieve structures are suggestedincluding the MFI, MEL, EUO, FER, MFS, MTT, MTW, TON, MOR and FAU typesof zeolites. The catalysts also contain a platinum group metal which mayexist in the catalyst as the metal or as a compound such as an oxide,sulfide, halide or oxysulfide. U.S. Pat. Nos. 3,856,872; 4,899,011;4,939,110 and 6,797,849 disclose, inter alia, MTW-type zeolites forxylene isomerization wherein the catalysts can contain at least onehydrogenation catalyst component.

Desirably the isomerization process as close to equilibrium as practicalin order to maximize the para-xylene yield; however, associated withthis is a greater cyclic C₈ loss due to side reactions. The approach toequilibrium that is used is an optimized compromise between high C₈cyclic loss at high conversion (i.e., very close approach toequilibrium) and high utility costs due to the large recycle rate ofunconverted C₈ aromatics. Catalysts thus are evaluated on the basis of afavorable balance of activity, selectivity and stability.

Due to the large scale of commercial facilities to produce para-xyleneon an economically competitive basis, not only must a xyleneisomerization process be active and stable, but it also must not undulycrack the aromatic feed so as to result in ring loss. Moreover, theisomerization processes produce by-products such as benzene, toluene,and aromatics having 9 or more carbon atoms. For instance, U.S. Pat. No.6,872,866 discloses a liquid phase process using two catalysts for theisomerization of xylenes and ethylbenzene. The catalysts comprise betazeolite and low Si/Al₂ MTW.

Often the xylene-containing feed to be isomerized also containsethylbenzene. Ethylbenzene may be dealkylated such as would occur in theprocesses of U.S. Pat. No. 6,872,866, or the ethylbenzene can beconverted. Advantageously, isomerization processes would convertethylbenzene to xylenes. Whether the isomerization process willdealkylate or will convert ethylbenzene depends upon the isomerizationconditions including catalyst.

Catalysts for isomerization of C₈ aromatics ordinarily are classified bythe manner of processing ethylbenzene associated with the xyleneisomers. Ethylbenzene is not easily isomerized to xylenes, but isnormally converted in the isomerization unit because separation from thexylenes by superfractionation or adsorption is very expensive. A widelyused approach is to dealkylate ethylbenzene to form principally benzenewhile isomerizing xylenes to a near-equilibrium mixture. An alternativeapproach is to react the ethylbenzene to form a xylene mixture viaconversion to and reconversion from naphthenes in the presence of asolid acid catalyst with a hydrogenation-dehydrogenation function. Theformer approach commonly results in higher ethylbenzene conversion, thuslowering the quantity of recycle to the para-xylene recovery unit andconcomitant processing costs, but the latter approach enhances xyleneyield by forming xylenes from ethylbenzene. A catalyst composite andprocess which enhance conversion according to the latter approach, i.e.,achieve ethylbenzene isomerization to xylenes with high conversion,would effect significant improvements in xylene-production economics.

Although numerous proposals have been made for catalyst andisomerization reactor schemes to achieve desired ethylbenzene conversionand xylene isomerization, the catalytic isomerization processes that arein commercial practice included both those that dealkylate and thosethat convert ethylbenzene. The commercially available catalysts forthese isomerization processes are believed to be based upon EUO or ZSM-5or MOR type zeolites in association with a hydrogenating catalyticcomponent. Advantageously, improved catalysts that are capable ofconverting ethylbenzene would be capable of being readily retrofittedinto such commercial processes.

SUMMARY OF THE INVENTION

The present invention provides a process for the isomerization ofalkylaromatic hydrocarbons. More specifically, the process of thepresent invention is directed to C₈ aromatic hydrocarbons isomerizationover certain catalysts containing MTW-type zeolite in order to convertethylbenzene to xylenes and to obtain improved yields of desired xyleneisomers.

The present invention is based on the discovery that a catalyst systemcomprising a low Si/Al₂ MTW-type zeolite, preferably substantiallymordenite-free, with at least one hydrogenation catalytic component, andpreferably a binder, demonstrates improved conversion and selectivity inC₈ aromatics isomerization, while minimizing undesired benzeneformation. The catalyst may further comprise a Group IVA (IUPAC 14)component such as tin. Advantageously, the processes can benefit throughlow ring loss while still achieving desirable conversions ofethylbenzene and approaches to xylene equilibrium. Further thosearomatic by-products formed during the isomerization tend to be thosethat can readily be converted to xylenes such as toluene and C₉ and C₁₀aromatics.

The broad aspects of the processes of this invention comprise contactinga feed stream containing a non-equilibrium admixture of at least onexylene isomer and ethylbenzene wherein preferably between about 1 and60, and more frequently between about 5 and 35, mass-% of the feedstream is ethylbenzene with a catalyst comprising MTW type zeolitehaving a silica/alumina mole ratio of between about 20:1 and 45:1 and acatalytically effective amount of at least one hydrogenation catalystcomponent, preferably a platinum group metal-containing component, underisomerization conditions. The isomerization conditions include thepresence of hydrogen in a mole ratio to hydrocarbon of at least about0.5:1, say, 0.5 to 6:1, preferably 1.5:1 to 5:1. Preferably, the feedstream contains naphthenes, and more preferably a sufficientconcentration of naphthenes is provided in the feed stream to enhancethe conversion of ethylbenzene, e.g., between about 2 and 20 mass-%naphthenes. Preferably, the isomerization is conducted under at leastpartially vapor phase conditions.

DETAILED DESCRIPTION OF THE INVENTION

The feedstocks to the aromatics isomerization processes of thisinvention comprise isomerizable alkylaromatic hydrocarbons of thegeneral formula C₆H_((6-n))R_(n), where n is an integer from 2 to 5 andR is CH₃, C₂H₅, C₃H₇, or C₄H₉, in any combination and including all theisomers thereof. Suitable alkylaromatic hydrocarbons include, forexample but without so limiting the invention, ortho-xylene,meta-xylene, para-xylene, ethylbenzene, ethyltoluenes,tri-methylbenzenes, diethylbenzenes, triethylbenzenes,methylpropylbenzenes, ethylpropylbenzenes, di-isopropylbenzenes, andmixtures thereof.

A particularly preferred application of the catalyst system of thepresent invention is the isomerization of a C₈ aromatic mixturecontaining ethylbenzene and xylenes. Generally the mixture will have anethylbenzene content of about 1 to about 60, preferably, about 1 toabout 50 wt-%; an ortho-xylene content of 0 to about 35 wt-%; ameta-xylene content of about 20 to about 95 wt-% and a para-xylenecontent of 0 to about 30 wt-%. The C₈ aromatics are a non-equilibriummixture, i.e., at least one C₈ aromatic isomer is present in aconcentration that differs substantially from the equilibriumconcentration at isomerization conditions. Usually the non-equilibriummixture is prepared by removal of para-, ortho- and/or meta-xylene froma fresh C₈ aromatic mixture obtained from an aromatics-productionprocess.

The alkylaromatic hydrocarbons may be utilized in the present inventionas found in appropriate fractions from various refinery petroleumstreams, such as individual components or as certain boiling-rangefractions obtained by the selective fractionation and distillation ofcatalytically cracked or reformed hydrocarbons. The process of thepresent invention allows the isomerization of alkylaromatic-containingstreams such as catalytic reformate with or without subsequent aromaticsextraction to produce specified xylene isomers and particularly toproduce para-xylene.

A C₈ aromatics feed to the present process may contain nonaromatichydrocarbons, i.e., naphthenes and paraffins, in an amount up to about30 wt-%, and preferably contains naphthenes in an amount sufficient toenhance the ethylbenzene conversion. Naphthenes are cyclic paraffins andmay include, for purposes herein, cyclic compounds having non-aromaticunsaturation in the ring structure. A convenient source of naphthenes isthe isomerization process itself which produces naphthenes. Typicallythe naphthenes that are recycled are monocyclic compounds, especially 5and 6 carbon atom rings, having from 5 to 9 carbon atoms. The downstreamunit operations will define the composition and amount of naphthenesbeing recycled. Generally, the naphthenes are present in an amount ofabout 2 to 20, preferably from about 4 to 15, wt-% of the feed.Equilibria may exist under isomerization conditions between naphthenesand aromatics. Thus, at isomerization conditions that convert a greaterpercentage of ethylbenzene, greater concentrations of naphthenes arepreferred. As the naphthenes are a by-product of the isomerization,usually the isomerization unit is started up with the xylene andethylbenzene feed and then the sought amount of naphthenes are permittedto build up for steady-state operation.

Preferably, the isomerizable hydrocarbons consist essentially ofaromatics, to ensure pure products from downstream recovery processes.Moreover, a C₈ aromatics feed that is rich in undesired ethylbenzene canbe supplied such that it can be converted to xylenes.

According to the process of the present invention, an alkylaromatichydrocarbon feed mixture, in the presence of hydrogen, is contacted witha catalyst of the type described below in an alkylaromatic hydrocarbonisomerization zone. Contacting may be effected using the catalyst in afixed-bed system, a moving-bed system, a fluidized-bed system, or in abatch-type operation. The fixed bed operation is preferred due to theease of operation and reduced attrition loss of the valuable catalyst.In this system, a hydrogen-rich gas and the feed mixture are preheatedby suitable heating means to the desired reaction temperature and thenpassed into an isomerization zone containing a fixed bed of catalyst.The isomerization zone may be one or more separate reactors withsuitable means therebetween to ensure that the desired isomerizationtemperature is maintained at the entrance to each reactor. The reactantsmay be contacted with the catalyst bed in either upward-, downward-, orradial-flow fashion, and the reactants may be in the liquid phase, amixed liquid-vapor phase, or a vapor phase when contacted with thecatalyst.

The alkylaromatic feed mixture, preferably a non-equilibrium mixture ofC₈ aromatics, is contacted with the isomerization catalyst at suitablealkylaromatic-isomerization conditions. Such conditions comprise atemperature ranging from about 0° to about 600° C. or more, generallywithin the range of about 100° to about 500° C. or more, and preferablyin the range from about 150° to 450° C., or from about 300° to about500° C. The pressure generally is from about 1 to 100 atmospheresabsolute, or may be less than about 50 atmospheres, say, about 10 kPa toabout 5 MPa absolute, preferably from about 100 kPa to about 3 MPaabsolute. Sufficient catalyst is contained in the isomerization zone toprovide a liquid hourly space velocity with respect to the hydrocarbonfeed mixture of from about 0.1 to 30 hr⁻¹, and preferably 0.5 to 10 h⁻¹.The hydrocarbon feed mixture optimally is reacted in admixture withhydrogen at a hydrogen/hydrocarbon mole ratio of about 0.5:1 to about25:1 or more, say, between about 0.5:1 to 6:1, preferably about 1.5:1 to5:1. One of the advantages of the processes of this invention is thatrelatively low partial pressures of hydrogen are still able to providethe sought selectivity and activity of the isomerization andethylbenzene conversion. Other inert diluents such as nitrogen, argonand light hydrocarbons may be present.

The isomerization conditions may be such that the isomerization isconducted in the liquid, vapor or at least partially vaporous phase. Forconvenience in hydrogen distribution, the isomerization is preferablyconducted in at least partially in the vapor phase. When conducted atleast partially in the vaporous phase, the partial pressure of C₈aromatics in the reaction zone is preferably such that at least about 50mass-% of the C₈ aromatics would be expected to be in the vapor phase.Often the isomerization is conducted with essentially all the C₈aromatics being in the vapor phase.

The reaction proceeds via the mechanism of isomerizing xylenes whilereacting ethylbenzene to form a xylene mixture via conversion to andreconversion from naphthenes. The yield of xylenes in the product isenhanced by forming xylenes from ethylbenzene. The loss of C₈ aromaticsthrough the reaction is low: typically less than about 4 wt-% per passof C₈ aromatics in the feed to the reactor, preferably no more thanabout 3.5 wt-%, and most preferably less than 3 wt-%.

Usually the isomerization conditions are sufficient that at least about10%, preferably between about 20 and 50%, of the ethylbenzene in thefeed stream is converted. Generally the isomerization conditions do notresult in a xylene equilibrium being reached. Often, the mole ratio ofxylenes in the product stream is at least about 80%, say between about85 and 95%, of equilibrium under the conditions of the isomerization.Where the isomerization process is to generate para-xylene, e.g., frommeta-xylene, the feed stream contains less than 5 mass-% para-xylene andthe isomerization product comprises a para-xylene/xylenes mole ratio ofbetween about 0.20:1 to 0.25:1.

The particular scheme employed to recover an isomerized product from theeffluent of the reactors of the isomerization zone is not critical tothe instant invention, and any effective recovery scheme known in theart may be used. Typically, the liquid product is fractionated to removelight and/or heavy byproducts to obtain the isomerized product. Heavybyproducts include A₁₀ compounds such as dimethylethylbenzene. In someinstances, certain product species such as ortho-xylene ordimethylethylbenzene may be recovered from the isomerized product byselective fractionation. The product from isomerization of C₈ aromaticsusually is processed to selectively recover the para-xylene isomer,optionally by crystallization or by selective adsorption usingcrystalline aluminosilicates according to U.S. Pat. No. 3,201,491,hereby incorporated herein by reference. Alternative adsorption recoveryprocesses are described in U.S. Pat. Nos. 3,626,020; 3,696,107;4,039,599; 4,184,943; 4,381,419 and 4,402,832, incorporated herein byreference.

The catalysts used in the processes of this invention comprise a lowSi/Al₂ MTW type zeolite, also characterized as “low silica ZSM-12”, anddefined in the instant invention to include molecular sieves with asilica/alumina ratio less than about 45, preferably from about 20 toabout 40 or 45, and sometimes 25 to 40. The preparation of the preferredMTW-type zeolites by crystallizing a mixture comprising an aluminasource, a silica source and templating agent using methods well known inthe art. U.S. Pat. No. 3,832,449, which is herein incorporated byreference, more particularly describes an MTW-type zeolite usingtetraalkylammonium cations. U.S. Pat. No. 4,452,769 and 4,537,758, whichare incorporated herein by reference, use a methyltriethylammoniumcation to prepare a highly siliceous MTW-type zeolite. U.S. Pat. No.6,652,832 uses a N,N-dimethylhexamethyleneimine cation as a template toproduce low silica/alumina ratio MTW type zeolite without MFIimpurities. Preferably, high purity crystals are used as seeds forsubsequent batches. As stated in the foregoing patents, the originalcations of the as-synthesized material can be replaced in accordancewith techniques well known in the art, at least in part, by ion exchangewith other cations. Preferred replacing cations include hydrogen ionsand hydrogen precursor, e.g., ammonium ions and mixtures thereof.

The MTW-type zeolitic components of the catalysts of the presentinvention is preferably substantially mordenite-free. Substantiallymordenite-free is herein defined to mean a MTW component containing lessthan about 20 wt-% mordenite impurity, preferably less than about 10wt-%, and most preferably less than about 5 wt-% mordenite which isabout at the lower level of detectability using most characterizationmethods known to those skilled in the art such as X-ray diffractioncrystallography. Mordenite can form concurrently with the synthesis oflow Si/Al₂ MTW. Especially where the silica/alumina ratio of the MTW islowered, and the concomitant mordenite phase under low silica conditionsis minimized, a catalyst composite with excellent properties for lowaromatic ring loss when converting ethylbenzene to para-xylene underwith minimum benzene by-product production.

The MTW-type zeolite is preferably composited with a binder forconvenient formation of catalyst particles. The proportion of zeolite inthe catalyst may range from about 1 to about 99 wt-%, and is often about1 to about 90 wt-%, preferably about 2 to about 60 wt-%, and sometimes,2 to about 20 wt-%, the remainder other than metal and other componentsdiscussed herein being the binder component. Refractory inorganic oxidebinders are preferred and the binder should be a porous, adsorptivesupport having a surface area of about 25 to about 500 m²/g. Suitablebinder materials include those which have traditionally been used inhydrocarbon conversion catalysts such as: (1) refractory inorganicoxides such as aluminas, titania, zirconia, chromia, zinc oxide,magnesia, thoria, boria, silica-alumina, silica-magnesia,chromia-alumina, alumina-boria, silica-zirconia, phosphorus-alumina,etc.; (2) ceramics, porcelain, bauxite; (3) silicas or silica gel,silicon carbide, clays and silicates including those syntheticallyprepared and naturally occurring, which may or may not be acid treated,for example, attapulgite clay, diatomaceous earth, fuller's earth,kaolin, kieselguhr, etc.; (4) crystalline zeolitic aluminosilicates,either naturally occurring or synthetically prepared such as FAU, MEL,MFI, MOR, MTW (IUPAC Commission on Zeolite Nomenclature), in hydrogenform or in a form which has been exchanged with metal cations, (5)spinels such as MgAl₂O₄, FeAl₂O₄, ZnAl₂O₄, CaAl₂O₄, and other likecompounds having the formula MOAl₂O₃ where M is a metal having a valenceof 2; and (6) combinations of materials from one or more of thesegroups.

A preferred refractory inorganic oxide for use in the present inventionis alumina. Suitable alumina materials are the crystalline aluminasknown as the gamma-, eta-, and theta-alumina, with gamma- or eta-aluminaproviding the best results.

A suitable shape for the catalyst composite is an extrudate. Thewell-known extrusion method initially involves mixing of the molecularsieve with optionally the binder and a suitable peptizing agent to forma homogeneous dough or thick paste having the correct moisture contentto allow for the formation of extrudates with acceptable integrity towithstand direct calcination. Extrudability is determined from ananalysis of the moisture content of the dough, with a moisture contentin the range of from about 30 to about 50 wt-% being preferred. Thedough is then extruded through a die pierced with multiple holes and thespaghetti-shaped extrudate is cut to form particles in accordance withtechniques well known in the art. A multitude of different extrudateshapes is possible, including, but not limited to, cylinders,cloverleaf, dumbbell and symmetrical and asymmetrical polylobates. It isalso within the scope of this invention that the extrudates may befurther shaped to any desired form, such as spheres, by marumerizationor any other means known in the art.

Another suitable shape of the composite is a sphere continuouslymanufactured by the well-known oil drop method. Preparation ofalumina-bound spheres generally involves dropping a mixture of molecularsieve, alumina sol, and gelling agent into an oil bath maintained atelevated temperatures. Alternatively, gelation of a silica hydrosol maybe effected using the oil-drop method. One method of gelling thismixture involves combining a gelling agent with the mixture and thendispersing the resultant combined mixture into an oil bath or towerwhich has been heated to elevated temperatures such that gelation occurswith the formation of spheroidal particles. The gelling agents that maybe used in this process are hexamethylene tetraamine, urea or mixturesthereof. The gelling agents release ammonia at the elevated temperatureswhich sets or converts the hydrosol spheres into hydrogel spheres. Thespheres are then continuously withdrawn from the oil bath and typicallysubjected to specific aging treatments in oil and an ammoniacal solutionto further improve their physical characteristics.

Preferably, the resulting composites are then washed and dried at arelatively low temperature of about 50° to 200° C. and subjected to acalcination procedure at a temperature of about 450° to 700° C. for aperiod of about 1 to about 20 hours.

Catalysts of the invention also comprise a hydrogenation catalystcomponent, especially a platinum-group metal, including one or more ofplatinum, palladium, rhodium, ruthenium, osmium, and iridium. Thepreferred platinum-group metal is platinum. The platinum-group metalcomponent may exist within the final catalyst composite as a compoundsuch as an oxide, sulfide, halide, oxysulfide, etc., or as an elementalmetal or in combination with one or more other ingredients of thecatalyst composite. It is believed that the best results are obtainedwhen substantially all the platinum-group metal component exists in areduced state. This component may be present in the final catalystcomposite in any amount which is catalytically effective; theplatinum-group metal generally will comprise about 0.01 to about 2 wt-%of the final catalyst, calculated on an elemental basis. Excellentresults are obtained when the catalyst contains about 0.05 to about 1wt-% of platinum.

The platinum-group metal component may be incorporated into the catalystcomposite in any suitable manner. One method of preparing the catalystinvolves the utilization of a water-soluble, decomposable compound of aplatinum-group metal to impregnate the calcined sieve/binder composite.Alternatively, a platinum-group metal compound may be added at the timeof compositing the sieve component and binder. Complexes of platinumgroup metals which may be employed in impregnating solutions,co-extruded with the sieve and binder, or added by other known methodsinclude chloroplatinic acid, chloropalladic acid, ammoniumchloroplatinate, bromoplatinic acid, platinum trichloride, platinumtetrachloride hydrate, platinum dichlorocarbonyl dichloride, tetramineplatinic chloride, dinitrodiaminoplatinum, sodium tetranitroplatinate(II), palladium chloride, palladium nitrate, palladium sulfate,diaminepalladium (II) hydroxide, tetraminepalladium (II) chloride, andthe like. It is within the scope of the present invention that thecatalyst composites may contain other metal components. Such metalmodifiers may include rhenium, tin, germanium, lead, cobalt, nickel,indium, gallium, zinc, uranium, dysprosium, thallium, and mixturesthereof. Catalytically effective amounts of such metal modifiers may beincorporated into the catalysts by any means known in the art to effecta homogeneous or stratified distribution.

A Group IVA (IUPAC 14) metal component is an optional ingredient of thecatalyst of the present invention. Of the Group IVA (IUPAC 14) metals,germanium and tin are preferred and tin is especially preferred. Thiscomponent may be present as an elemental metal, as a chemical compoundsuch as the oxide, sulfide, halide, oxychloride, etc., or as a physicalor chemical combination with the porous carrier material and/or othercomponents of the catalyst. Preferably, a substantial portion of theGroup IVA (IUPAC 14) metal exists in the finished catalyst in anoxidation state above that of the elemental metal. The Group IVA (IUPAC14) metal component optimally is utilized in an amount sufficient toresult in a final catalyst containing about 0.01 to about 5 wt-% metal,calculated on an elemental basis, with best results obtained at a levelof about 0.1 to about 2 wt-% metal.

The optional Group IVA (IUPAC 14) metal component may be incorporated inthe catalyst in any suitable manner to achieve a homogeneous dispersion,such as by coprecipitation with the porous carrier material,ion-exchange with the carrier material or impregnation of the carriermaterial at any stage in the preparation. One method of incorporatingthe Group IVA (IUPAC 14) metal component into the catalyst compositeinvolves the utilization of a soluble, decomposable compound of a GroupIVA (IUPAC 14) metal to impregnate and disperse the metal throughout theporous carrier material. The Group IVA (IUPAC 14) metal component can beimpregnated either prior to, simultaneously with, or after the othercomponents are added to the carrier material. Thus, the Group IVA (IUPAC14) metal component may be added to the carrier material by comminglingthe latter with an aqueous solution of a suitable metal salt or solublecompound such as stannous bromide, stannous chloride, stannic chloride,stannic chloride pentahydrate; or germanium oxide, germaniumtetraethoxide, germanium tetrachloride; or lead nitrate, lead acetate,lead chlorate and the like compounds. The utilization of Group IVA(IUPAC 14) metal chloride compounds, such as stannic chloride, germaniumtetrachloride or lead chlorate is particularly preferred since itfacilitates the incorporation of both the metal component and at least aminor amount of the preferred halogen component in a single step. Whencombined with hydrogen chloride during the especially preferred aluminapeptization step described hereinabove, a homogeneous dispersion of theGroup IVA (IUPAC 14) metal component is obtained in accordance with thepresent invention. In an alternative embodiment, organic metal compoundssuch as trimethyltin chloride and dimethyltin dichloride areincorporated into the catalyst during the peptization of the inorganicoxide binder, and most preferably during peptization of alumina withhydrogen chloride or nitric acid.

The catalysts of the present invention may contain a halogen component,comprising either fluorine, chlorine, bromine or iodine or mixturesthereof, with chlorine being preferred. Preferably, however, thecatalyst contains no added halogen other than that associated with othercatalyst components.

The catalyst composite is dried at a temperature of from about 100° toabout 320° C. for a period of from about 2 to about 24 or more hoursand, usually, calcined at a temperature of from about 400° to about 650°C. in an air atmosphere for a period of from about 0.1 to about 10 hoursuntil the metallic compounds present are converted substantially to theoxide form. If desired, the optional halogen component may be adjustedby including a halogen or halogen-containing compound in the airatmosphere.

The resultant calcined composites optimally are subjected to asubstantially water-free reduction step to ensure a uniform and finelydivided dispersion of the optional metallic components. The reductionoptionally may be effected in the process equipment of the presentinvention. Substantially pure and dry hydrogen (i.e., less than 20 vol.ppm H₂O) preferably is used as the reducing agent in this step. Thereducing agent contacts the catalyst at conditions, including atemperature of from about 200° to about 650° C. and for a period of fromabout 0.5 to about 10 hours, effective to reduce substantially all ofthe Group VIII metal component to the metallic state. In some cases theresulting reduced catalyst composite may also be beneficially subjectedto presulfiding by a method known in the art such as with neat H₂S atroom temperature to incorporate in the catalyst composite from about0.05 to about 1.0 wt-% sulfur calculated on an elemental basis.

EXAMPLES

The following examples are presented only to illustrate certain specificembodiments of the invention, and should not be construed to limit thescope of the invention as set forth in the claims. There are manypossible other variations within the spirit of the invention, as thoseof ordinary skill in the art will recognize.

Example I

Samples of catalysts comprising zeolites are prepared for comparativepilot-plant testing. First, a Catalyst A is prepared to represent aprior art catalyst for use in a process of isomerization of ethylbenzeneto para-xylene with minimal benzene formation

Catalyst A contains SM-3 silicoaluminophosphate such as disclosed inU.S. Pat. No. 4,943,424, hereby incorporated by reference, and hascharacteristics as disclosed in the '424 patent. Following the teachingsof U.S. Pat. No. 5,898,090, hereby incorporated by reference, Catalyst Ais composited with hydrous alumina and tetramine platinic chloride at aplatinum level of 0.4 wt-% on an elemental basis. The compositecomprises about 60 wt-% SM-3 and 40 wt-% alumina. The catalyst iscalcined and reduced, with the product labeled as Catalyst A.

Example II

Catalysts are prepared containing MTW-type zeolite prepared inaccordance with U.S. Pat. No. 4,452,769. To a solution of 0.2 mass-partssodium hydroxide in 9 mass-parts distilled water are added 0.195mass-parts aluminum hydroxide hydrate and stirred until dissolved. Asecond solution of 1.5 mass-parts of methyltriethylammonium chloride in9 mass-parts distilled water is prepared and stirred until dissolved.Then, both solutions are stirred together until homogenized. Next, 3mass-parts of precipitated silica are added, stirred for 1 hour at roomtemperature and sealed in a Teflon-lined autoclave for 8 days at 150° C.Zeolite type MTW is recovered after cooling, filtering, and washing withdistilled water. After drying, the recovered product is calcined at 550°C. to remove the template and ion-exchanged three times with NH₄NO₃ anddried to show the following analysis: 0.9NH₄:Al₂O₃:41SiO₂:84H₂O. TheX-ray diffraction pattern is consistent with an MTW structure zeoliteand no mordenite phase is detected.

To form Catalyst B, about 10 wt % of this MTW-zeolite (dry) iscomposited with about 90 wt-% alumina to form extruded shaped catalystparticles. The particles are then metal-impregnated using a solution ofchloroplatinic acid. Upon completion of the impregnation, the catalystis dried, oxidized, reduced, and sulfided to yield a catalyst containingabout 0.3 wt-% platinum and 0.1 wt-% sulfur. The finished catalyst islabeled Catalyst B.

Example III

Catalysts A and B are evaluated for ethylbenzene isomerization topara-xylene using a pilot plant flow reactor processing anon-equilibrium C₈ aromatic feed having the following approximatecomposition in wt-%:

Toluene 0.2 C₈ Non-aromatics 8.3 Ethylbenzene 26.8 Para-xylene 0.9Meta-xylene 42.4 Ortho-xylene 21.0 C₉ ⁺ Non-aromatics 0.4

This feed is contacted with catalyst at a pressure of about 620 kPa, aliquid hourly space velocity of 3 hr⁻¹, and a hydrogen/hydrocarbon moleratio of 4. Reactor temperature is adjusted to effect a favorableconversion level. Conversion is expressed as the disappearance per passof ethylbenzene, and C₈ aromatic ring loss is primarily to benzene andtoluene, with smaller amounts of light gases being produced. Results areas follows:

Catalyst A B Temperature ° C. 386 371 p-xylene/xylenes 22.5 22.3 EBconversion, wt % 31 38 Benzene yield, wt % 0.25 0.10 C₈ Ring loss 2.52.5

Catalyst B shows better conversion of ethylbenzene while minimizing theyield of undesired benzene as compared to Catalyst A. The “C₈ ring loss”is in units of mol-% defined as “(1−(C₈ naphthenes and aromatics inproduct)/(C₈ naphthenes and aromatics in feed))*100”, which representsmaterial that is to be circulated to another unit in an aromaticscomplex. Such circulation is expensive and a low amount of C₈ ring lossis a favorable feature of the catalyst of the present invention.

Example IV

Similarly, additional batches of MTW-type zeolite are prepared accordingthe procedure outlined above in Example II. Due to variations instirring and seed crystals as well as other inhomogeneous effects amongthe vessels used, resulting batches have various amounts of impuritiesat a silica/alumina ratio of about 34. Using X-ray diffraction methods,the impurities are determined to be a mordenite-type zeolite. Tounderstand the effect of the impurity, various samples are obtained andmade into catalysts as described below.

Catalyst C is prepared with the same material as Catalyst B, 100 wt-%MTW, see Example II. Catalyst D is prepared with a zeolitic compositecomprising 90 wt-% MTW and 10 wt-% mordenite. Catalyst E is preparedwith a zeolitic composite comprising 80 wt-% MTW and 20 wt-% mordenite.Finally, Catalyst F is prepared with a zeolitic composite comprising 50wt-% MTW and 50 wt-% mordenite to illustrate a catalyst with substantialmordenite impurity and thus is not considered a catalyst within thescope of the invention.

Catalysts C through F are formed into extruded particles using about 5wt-% of the zeolitic composite material above and about 95 wt-% aluminabinder. The particles are then metal-impregnated using a solution ofchloroplatinic acid. Upon completion of the impregnation, the catalystsare dried, oxidized, reduced, and sulfided to yield catalysts containingabout 0.3 wt-% platinum and 0.1 wt-% sulfur. The finished catalysts arelabeled respectively, Catalysts C through F.

Example V

Catalysts C through F are evaluated for C₈ aromatic ring loss using apilot plant flow reactor processing a non-equilibrium C₈ aromatic feedhaving the following approximate composition in wt-%:

C₈ Non-aromatics 7 Ethylbenzene 16 Para-xylene <1 Meta-xylene 52Ortho-xylene 25

This feed is contacted with a catalyst at a pressure of about 620 kPa, aliquid hourly space velocity of 4 hr⁻¹, and a hydrogen/hydrocarbon moleratio of 4. Reactor temperature is adjusted between about 370° to 375°C. to effect a favorable ethylbenzene conversion level. Results are asfollows:

Catalyst C D E F p-xylene/xylenes 22.3 22.3 22.3 22.3 C₈ Ring loss 2.63.3 3.6 5.4

Catalyst C shows minimum ring loss, and Catalysts D thru F illustratethat mol-% ring loss increases with mordenite impurity level. A lowamount of C₈ ring loss is a favorable feature of the catalysts of thepresent invention, which contain MTW-type zeolite substantially free ofthe mordenite impurity.

Example VI

Catalyst G is prepared to illustrate a bimetallic catalyst of thepresent invention. Catalyst G is prepared with the same zeoliticmaterial of Catalyst B, 100 wt-% MTW type zeolite (see Example II), andformed into extruded particles using about 5 wt-% of the zeoliticmaterial and about 95 wt-% alumina binder. The particles are thenmetal-impregnated using a first aqueous solution of tin chloride in acold rolling evaporative impregnation vessel for about 1 hour and thensteamed to dryness. The tin-impregnated base is calcined at 550° C. inair for 2 hours.

Then a second aqueous platinum impregnation is conducted withchloroplatinic acid and similarly cold rolled for 1 hour and steamed todryness. The catalyst is then oxidized and reduced to produce a finishedcatalyst containing about 0.3 wt-% of platinum and about 0.1 wt-% oftin, which is labeled as Catalyst G.

Example VII

Catalysts B and G are evaluated for stability in ethylbenzeneisomerization to para-xylene using a pilot plant flow reactor processinga non-equilibrium C₈ aromatic feed having the same approximatecomposition as Example III above. This feed is contacted with catalystat a pressure of about 690 kPa, a weight hourly space velocity of about9.5 hr⁻¹, and a hydrogen/hydrocarbon mole ratio of 4. Reactortemperature is set at 385° C. and conversion is allowed to decline overtime.

Results show that Catalyst G has about a 5 wt-% lower initial conversionof ethylbenzene when compared to Catalyst B, but that Catalyst G has adeactivation rate that is only about two-thirds that of Catalyst B.Deactivation rate is determined based on the rate of decline ofethylbenzene conversion over time under the test conditions above.

When a second comparative test is conducted at the same conditions asabove except using a 3 hr⁻¹ weight hourly space velocity, theethylbenzene conversion performance of Catalyst G exceeds theperformance of Catalyst B after about 130 hours on stream. Thus,Catalyst G shows that superior stability, in terms of decreaseddeactivation, provides long term value for the isomerization ofethylbenzene into xylenes and that increased yields are produced whenconversion is averaged over an extended time period. Moreover, it shouldbe noted that the catalyst performance in terms of C₈ ring loss is aboutequivalent between Catalyst B and Catalyst G.

Example VIII

Samples of catalyst are prepared.

Catalyst H: To a solution of 0.2 mass-parts sodium hydroxide in 9mass-parts distilled water are added 0.195 mass-parts aluminum hydroxidehydrate and stirred until dissolved. A second solution of 1.5 mass-partsof methyltriethylammonium chloride in 9 mass-parts distilled water isprepared and stirred until dissolved. Then, both solutions are stirredtogether until homogenized. Next, 3 mass-parts of precipitated silicaare added, stirred for 1 hour at room temperature and sealed in aTeflon-lined autoclave for 9 days at 150° C. Zeolite type MTW isrecovered after cooling, filtering, and washing with distilled water.After drying, the recovered product is an MTW-type zeolite having thefollowing analysis: 0.24Na₂O:1.0Al₂O₃:34SiO₂:55H₂O:1.2(MTEA-CI). TheX-ray diffraction pattern is consistent with an MTW structure zeolite.

The as-synthesized zeolite powder is admixed with a hydrous aluminabinder to provide a composite of 5 mass-parts of zeolite to 95mass-parts of alumina. The composite is extruded to form pellets. Theextrudate is calcined in air at 565° C. for 4 hours. The pellets arethen ion-exchanged at 88° C. with a 15 mass-% solution of NH₄NO₃,washed, dried and re-calcined in air at 565° C. The pellets are thenimpregnated with a solution of chloroplatinic acid with 3.5 mass-%hydrochloric acid to provide a final platinum level of 0.31 mass-% onthe final catalyst. The impregnated pellets are then oxidized andchloride adjusted at 565° C. to yield 1.22 mass-% chloride on thecatalyst, subjected to a reducing environment of hydrogen at 565° C.,and sulfided with hydrogen sulfide to yield 0.08 mass-% sulfur on thecatalyst.

Catalyst J (Comparative): The starting material is an EUO structurezeolite. See, for instance U.S. Pat. No. 6,057,486 for references formaking EUO-type zeolite. The ammonium-form of the zeolite is admixedwith a hydrous alumina binder to provide a composite of 10 mass-parts ofzeolite to 90 mass-parts of alumina. The composite is extruded to formpellets. The extrudate is calcined in air at 565° C. for 4 hours. Thepellets are then impregnated with a solution of chloroplatinic acid with2.0 mass-% hydrochloric acid to provide a final platinum level of 0.28mass-% on the final catalyst. The impregnated pellets are then oxidizedand chloride adjusted at 565° C. to yield 1.63 mass-% chloride on thecatalyst, subjected to a reducing environment of hydrogen at 565° C.,and sulfided with hydrogen sulfide to yield 0.09 mass-% sulfur on thecatalyst.

Catalyst K (Comparative): An SM-3, crystalline silicoaluminophosphatesuch as disclosed in U.S. Pat. No. 4,943,424 (Miller) is used to makeCatalyst C. The SM-3 is composited with hydrous alumina and tetra-ammineplatinum chloride. The composites comprise about 60 mass-% SM-3 and 40mass-% alumina. Tetra-ammine platinum chloride is incorporated into thecomposites to effect platinum and chloride contents of 0.39 and 0.21mass-%, respectively, on an elemental basis, and the catalyst iscalcined, reduced and sulfided to yield 0.07 mass-% sulfur.

Catalyst L (Comparative): A spherical catalyst comprising about 10mass-% MOR in an alumina matrix is prepared by oil dropping according tothe teachings of U.S. Pat. No. 2,620,314. The calcined, reduced andsulfided catalyst contains about 0.28 mass-% platinum, 0.86 mass-%chloride and 0.11 mass-% sulfur.

Example IX

Catalysts H, J, K and L are evaluated in a pilot plant for theisomerization of a feed stream containing 16 mass-% ethylbenzene, 25mass-% ortho-xylene and 52 mass-% meta-xylene. The feed contains 7mass-% naphthenes. The pilot plant runs are at a hydrogen/hydrocarbonratio of 4:1. The pilot plant runs are summarized in Table 1. Theproduct data for Catalysts H, J, and K are taken at approximately 200hours of operation. The data from Catalyst L are taken at 150 hours. Thehigher activity of Catalyst H requires a higher LHSV for purposes ofcomparison with Catalysts J, K and L at approximately the sametemperatures. The liquid hourly space velocities are based upon theliquid volume of the feed to the pilot plant.

TABLE 1 J K L Catalyst H Comparative Comparative Comparative LHSV, hr⁻¹3.9 2.9 3.0 3.0 WABT, ° C. 373 380 396 386 Pressure, kPa g 570 695 1020745 Para-xylene/xylene 22.3 22.5 22.4 22.2 EB Conversion, % 38 20 23 33C₈ loss, mass % 3.0 4.4 4.3 5.9 C₇ and C₉ aromatics, 52.5 34.1 41.7 50.4mass % of by-products C₆ and C₁₀ aromatics, 8.4 5.7 12.4 16.6 mass % ofby-products Non-C₈ paraffins and 39.1 60.2 45.9 33.0 naphthenes, mass %of by-products

The results illustrated in Table 1 demonstrate that Catalyst H, having alow Si/Al MTW-type zeolite, not only exhibits high activity but also lowring loss and importantly, a by-product composition that enhances therecovery of aromatic values. For instance, toluene is the feedstock fora disproportionation unit operation such as disclosed in U.S. Pat. Nos.4,016,219 and 4,097,543 to make xylenes. Toluene and C₉ aromatics can bea feed stock for a transalkylation unit operation such as disclosed inU.S. Pat. No. 4,341,914 to make xylenes.

Example X

Three MTW-containing catalysts are prepared having varying Si/Al₂ratios.

Catalyst M: To a solution of 0.2 mass-parts sodium hydroxide in 9mass-parts distilled water are added 0.195 mass-parts aluminum hydroxidehydrate and stirred until dissolved. A second solution of 1.5 mass-partsof methyltriethylammonium chloride in 9 mass-parts distilled water isprepared and stirred until dissolved. Then, both solutions are stirredtogether until homogenized. Next, 3 mass-parts of precipitated silicaare added, stirred for 1 hour at room temperature and sealed in aTeflon-lined autoclave for 8 days at 150° C. Zeolite type MTW isrecovered after cooling, filtering, and washing with distilled water.After drying, the recovered product is calcined at 550° C. to remove thetemplate and ion-exchanged three times with NH₄NO₃ and dried to providean MTW having the following analysis: 0.9NH₄:Al₂O₃:41 SiO₂:84H₂O. TheX-ray diffraction pattern is consistent with an MTW structure zeolite.

The ammonium form of the zeolite is admixed with a hydrous aluminabinder to provide a composite of 10 mass-parts of zeolite to 90mass-parts of alumina. The composite is extruded to form pellets. Theextrudate is calcined in air at 565° C. for 4 hours. The pellets arethen impregnated with a solution of chloroplatinic acid with 2.0 mass-%hydrochloric acid to provide a final platinum level of 0.32 mass-% onthe final catalyst. The impregnated pellets are then oxidized andchloride adjusted at 565° C. to yield 1.19 mass-% chloride on thecatalyst, subjected to a reducing environment of hydrogen at 565° C.,and sulfided with hydrogen sulfide to yield 0.07 mass-% sulfur on thecatalyst.

Catalyst N (Comparative): To a solution of 0.2 mass-parts sodiumhydroxide in 9 mass-parts distilled water are added 0.13 mass-partsaluminum hydroxide hydrate and stirred until dissolved. A secondsolution of 1.5 mass-parts of methyltriethylammonium chloride in 9mass-parts distilled water is prepared and stirred until dissolved.Then, both solutions are stirred together until homogenized. Next, 3mass-parts of precipitated silica are added, stirred for 1 hour at roomtemperature and sealed in a Teflon-lined autoclave for 8 days at 150° C.Zeolite type MTW is recovered after cooling, filtering, and washing withdistilled water. After drying, the recovered product is calcined at 550°C. to remove the template and ion-exchanged three times with NH₄NO₃ anddried to provide an MTW having the following analysis:1.4NH₄:Al₂O₃:62SiO₂:10H₂O. The X-ray diffraction pattern is consistentwith an MTW structure zeolite.

The ammonium form of the zeolite is admixed with a hydrous aluminabinder to provide a composite of 10 mass-parts of zeolite to 90mass-parts of alumina. The composite is extruded to form pellets. Theextrudate is calcined in air at 565° C. for 4 hours. The pellets arethen impregnated with a solution of chloroplatinic acid with 2.0 mass-%hydrochloric acid to provide a final platinum level of 0.31 mass-% onthe final catalyst. The impregnated pellets are then oxidized andchloride adjusted at 565° C. to yield 1.31 mass-% chloride on thecatalyst, subjected to a reducing environment of hydrogen at 565° C.,and sulfided with hydrogen sulfide to yield 0.09 mass-% sulfur on thecatalyst.

Catalyst P (Comparative): To a solution of 0.2 mass-parts sodiumhydroxide in 9 mass-parts distilled water are added 0.078 mass-partsaluminum hydroxide hydrate and stirred until dissolved. A secondsolution of 1.5 mass-parts of methyltriethylammonium chloride in 9mass-parts distilled water is prepared and stirred until dissolved.Then, both solutions are stirred together until homogenized. Next, 3mass-parts of precipitated silica are added, stirred for 1 hour at roomtemperature and sealed in a Teflon-lined autoclave for 8 days at 150° C.Zeolite type MTW is recovered after cooling, filtering, and washing withdistilled water. After drying, the recovered product is calcined at 550°C. to remove the template and ion-exchanged three times with NH₄NO₃ anddried to provide an MTW having the following analysis:1.5NH₄:1Al₂O₃:88SiO₂:15H₂O. The X-ray diffraction pattern is consistentwith an MTW structure zeolite.

The ammonium form of the zeolite is admixed with a hydrous aluminabinder to provide a composite of 10 mass-parts of zeolite to 90mass-parts of alumina. The composite is extruded to form pellets. Theextrudate is calcined in air at 565° C. for 4 hours. The pellets arethen impregnated with a solution of chloroplatinic acid with 2.0 mass-%hydrochloric acid to provide a final platinum level of 0.26 mass-% onthe final catalyst. The impregnated pellets are then oxidized andchloride adjusted at 565° C. to yield 1.23 mass-% chloride on thecatalyst, subjected to a reducing environment of hydrogen at 565° C.,and sulfided with hydrogen sulfide to yield 0.06 mass-% sulfur on thecatalyst.

Example XI

Catalysts M, N and P are evaluated in a pilot plant for theisomerization of a feed stream containing 27 mass-% ethylbenzene, 22mass-% ortho-xylene, 44 mass-% meta-xylene and 7 mass-% naphthenes. Thepilot plant runs are at 550 kPa gauge with a hydrogen/hydrocarbon ratioof 4:1. The pilot plant runs are summarized in Table 2. The product dataare taken at about 92 hours of operation. The liquid hourly spacevelocities are based upon the liquid volume of the feed to the pilotplant.

TABLE 2 N P Catalyst M Comparative Comparative LHSV, hr⁻¹ 2.9 3.0 3.0WABT, ° C. (initial) 367 368 368 Pressure, kPa g 550 550 550Para-xylene/xylene 22.3 22.0 22.0 EB Conversion, % 43 38 31 C₈ loss,mass % 4.1 4.3 4.9 C₇ and C₉ aromatics, 40.6 46.2 44.1 mass % ofby-products C₆ and C₁₀ aromatics, 7.6 12.0 14.0 mass % of by-productsNon-C₈ paraffins and 51.8 41.8 41.9 naphthenes, mass % of by-products

To maintain a constant para-xylene/xylene ratio at the given LHSV, thetemperature of the reactor for Catalyst P needs to be increased fromabout 368° to 388° C. over 100 to 300 hours on stream. During the sameperiod of time, the reaction temperature for Catalyst E needs to beincreased from 368° to 374° C.

The results in Table 2 demonstrate that the low Si/Al ratio MTW typezeolite catalysts used in the processes of this invention reduce ringloss but still have high activities and stabilities.

1. A process for converting ethylbenzene to xylenes and isomerizing xylene in a feed stream comprising ethylbenzene and a non-equilibrium mixture of one or more xylenes comprising contacting the feed stream with a catalyst comprising MTW type zeolite having a silica/alumina mole ratio of between about 20:1 and 45:1, from about 0.1 to about 2 wt-% of a platinum-group component calculated on an elemental basis, and from about 0.01 to about 5 wt-% of a tin component calculated on an elemental basis, under isomerization conditions said conditions including the presence of hydrogen in a mole ratio to hydrocarbon of at least about 0.5:1 said feed stream comprising between about 1 and 60 mass-% ethylbenzene and 2 to 20 mass-% naphthenes, to provide a conversion product.
 2. The process of claim 1 wherein between about 5 and 35 mass-% of the feed stream is ethylbenzene.
 3. The process of claim 1 wherein between about 20 and 50 percent of the ethylbenzene in the feed stream is converted.
 4. The process of claim 1 wherein the conversion product contains benzene in an amount of less than about 0.2 wt-% of the feed stream.
 5. The process of claim 1 wherein the hydrogen/hydrocarbon ratio is between about 1.5:1 to 6:1.
 6. A process for the isomerization of a feed mixture of xylenes and ethylbenzene comprising contacting the feed mixture with a catalyst comprising from about 0.1 to about 2 wt-% of a platinum-group component calculated on an elemental basis, from about 0.01 to about 5 wt-% of a tin component calculated on an elemental basis, from about 1 to about 99 wt-% of a substantially mordenite-free MTW-type zeolite component, having a silica/alumina mole ratio of about 45 or less, and an inorganic-oxide binder component at isomerization conditions comprising a temperature of from about 0° to 600° C., a pressure of from about 1 to 50 atmospheres, a liquid hourly space velocity of from about 0.1 to 30 hr⁻¹ and a hydrogen/hydrocarbon mole ratio of from about 0.5:1 to 25:1 to isomerize ethylbenzene to xylenes and obtain an isomerized product comprising a higher proportion of xylenes than in the feed mixture with a C₈ aromatics ring loss relative to the feed mixture no more than about 4 mol-%.
 7. The process of claim 6 wherein the zeolite silica/alumina ratio is in the range from about 20 to about
 40. 8. The process of claim 6 wherein the substantially mordenite-free MTW-type zeolite component comprises less than about 10 wt-% mordenite.
 9. The process of claim 6 wherein the substantially mordenite-free MTW-type zeolite component comprises less than about 5 wt-% mordenite.
 10. The process of claim 6 further comprising recovery of para-xylene by selective adsorption from the isomerized product.
 11. The process of claim 6 further comprising recovery of ortho-xylene from one or both of the isomerized product and the feed mixture.
 12. The process of claim 6 wherein the platinum-group metal is platinum.
 13. The process of claim 6 wherein the inorganic-oxide binder component is selected from the group consisting of alumina, silica, and mixtures thereof.
 14. The process of claim 6 wherein the isomerized product contains benzene in an amount of less than about 0.2 wt-% of the feed mixture.
 15. A process for the isomerization of a feed mixture of xylenes and ethylbenzene comprising contacting the feed mixture with a catalyst comprising from about 0.1 to about 2 wt-% of a platinum-group component calculated on an elemental basis, from about 1 to about 99 wt-% of a substantially mordenite-free MTW-type zeolite component, from about 0.01 to about 5 wt-% of a tin component calculated on an elemental basis, having a silica/alumina mole ratio of about 45 or less, and an inorganic-oxide binder component at isomerization conditions comprising a temperature of from about 0° to 600° C., a pressure of from about 1 to 50 atmospheres, a liquid hourly space velocity of from about 0.1 to 30 hr⁻¹ and a hydrogen/hydrocarbon mole ratio of from about 0.5:1 to 25:1 to isomerize ethylbenzene to xylenes and obtain an isomerized product comprising a higher proportion of xylenes than in the feed mixture with a C₈ aromatics ring loss relative to the feed mixture no more than about 4 mol-%. 